The present application relates to an optical device that utilizes an electrowetting phenomenon, and to an illumination apparatus and a camera that contain such an optical device.
Developments of optical devices that utilize an electrowetting phenomenon (electrocapillary phenomenon) have been made over the recent years. An electrowetting phenomenon refers to a change in the shape of a liquid surface that occurs when the energy at a solid-liquid interface between an electrode surface and an electrically conductive liquid changes in response to a voltage applied between the liquid and the electrode.
FIGS. 12A and 12B are principle diagrams illustrating an electrowetting phenomenon. As schematically shown in FIG. 12A, for example, an insulating film 202 is formed on the surface of an electrode 201, and an electrically conductive liquid droplet 203 of an electrolytic solution is set on the insulating film 202. The surface of the insulating film 202 is given a water-repellent treatment. Thus, when a voltage is not applied, a contact angle θ0 between the surface of the insulating film 202 and the liquid droplet 203 is large since interaction energy therebetween is low, as shown in FIG. 12A. The contact angle θ0 is an angle formed between the surface of the insulating film 202 and a tangent line of the liquid droplet 203 and depends on properties such as the surface tension of the liquid droplet 203 and the surface energy of the insulating film 202.
On the other hand, as schematically shown in FIG. 12B, when a voltage is applied between the electrode 201 and the liquid droplet 203, electrolyte ions in the liquid droplet 203 becomes concentrated near the surface of the insulating film 202 so as to cause a change in the charged amount of a charged double-layer, thereby inducing a change in the surface tension of the liquid droplet 203. This phenomenon is called an electrowetting phenomenon in which a contact angle θv of the liquid droplet 203 changes in accordance with the magnitude of the applied voltage. In other words, in FIG. 12B, the contact angle θv is a function of an applied voltage V and can be expressed by Lippman-Young's equation (A) below:cos(θv)=cos(θ0)+(1/2)(∈0·∈)/(γLG·t)×V2  (A)
where ∈0 denotes a dielectric constant in vacuum, ∈ denotes a relative dielectric constant of the insulating film 202, γLG denotes the surface tension of the electrolytic solution, and t denotes the film thickness of the insulating film 202.
Accordingly, the surface shape (curvature) of the liquid droplet 203 changes in accordance with the magnitude of voltage V applied between the electrode 201 and the liquid droplet 203. Therefore, when the liquid droplet 203 is used as a lens element, an optical element with an electrically controllable focal position (focal length) can be achieved.
Optical devices that employ such an optical element have been developed. For example, Japanese Unexamined Patent Application Publication No. 2000-356708 proposes a lens array for a stroboscopic apparatus. Regarding this lens array, an electrically conductive liquid and electrically insulating liquid droplets arranged in an array on a water-repellent film formed on the surface of a substrate are sealed so as to form variable focal-length lenses. The individual lenses are each formed by the shape of the interface between the electrically insulating liquid and the electrically conductive liquid, and the shape of each lens is electrically controlled by utilizing the electrowetting phenomenon so as to change the focal length. As another example, Japanese Unexamined Patent Application Publication No. 2002-162507 discloses a columnar lens formed of a liquid lens.